Dual optical angular encoder

ABSTRACT

To monitor the reliability of the information provided by a dual optical angular encoder, comprising two pairs of cells for detecting marks borne by a disc, at a given instant, the sequences of at least four successive states taken by the two groups of cells before this instant are compared and the incrementation or decrementation indications given by the two groups are validated if the sequence for a group is either identical or phase-offset by at most one state, ahead or behind, with respect to the sequence of the other group.

The invention relates to optical angular encoders providing binary logicsignals representing the increments of rotation of the encoder. Theseoptical encoders are used in the manner of potentiometers, for examplefor the manual control of electronic apparatuses sensitive to an inputparameter that may vary continuously or almost continuously, but theyare much more reliable than potentiometers. Typically, in an applicationin respect of aeronautical equipment, an optical angular encoder can beused to indicate to an automatic piloting computer an altitude or speedpreset that the pilot chooses by actuating a control button whichrotates the encoder. The reliability of the encoder and of theinformation that it delivers is then an essential element of theencoder.

It is desired to produce accurate and reliable encoders that are made tooperate in a secure manner so as to allow their basic function to beachieved even if certain elements of which they are composed develop afault.

An optical angular encoder typically consists of a disc bearing regularmarks, this disc being caused to rotate by actuation of a control button(for example a manual control button). A photoelectric cell fixed infront of the disc detects the filing past of the successive marks whenthe control button rotates the disc. The marks are typically openings inan opaque disc, a light-emitting diode being placed on one side of thedisc and the photoelectric cell being placed on the other side.

Each passage of a mark constitutes an increment of a unit in thecounting of the rotation of the disc. The angular resolution isdetermined by the angular pitch of the marks regularly disposed over adisc revolution. To detect both increments and decrements of angle ofrotation when the direction of rotation is reversed, two photoelectriccells are provided, physically offset by an odd number of quarters of apitch between them. Thus, the lit/unlit logic states of the two cellsare encoded on two bits which successively take the following foursuccessive values 00, 01, 11, 10 when the disc rotates in one directionand the following four successive values 00, 10, 11, 01 when the discrotates in the other, so that it is easy to determine, not only theappearance of an increment of rotation (change of state of one of thebits) but the direction of the increment (by comparison between a stateof the cells and the immediately prior state).

To increase the reliability of systems using such encoders, inparticular for aeronautical applications, it has been proposed that theencoder be dualized or at least that the photoelectric cells inside theencoder be dualized. This makes it possible in part to detect faultssuch as the nonoperation of a light-emitting diode or of a detectiondiode since the states provided by the two cells are compared and theincrementation or decrementation information is validated only if it isprovided in an identical manner by both encoders or both groups ofphotoelectric cells of the encoder. If the information is not identicalit may be concluded that one cell at least (light-emitting diode ordetection diode) is faulty and the count is invalidated by giving anerror or fault indication signal.

However, this comparison of the signals of the two groups of cells turnsout to be difficult since the positioning of the first group of cellsmust be rigorously identical to the positioning of the second group:while the offset by an odd number of quarters of a pitch of the marksbetween the two cells of one and the same group may be slightlyinaccurate, the offset between the two groups of cells must be veryaccurately a multiple of the pitch spacing of the marks.

If matters are not so, it leads to a situation in which theincrementations or decrementations determined by the cells do not occurrigorously at the same moment. Admittedly the increments and decrementswill be detected by the two groups of cells, but perhaps with a veryslight offset in time. Consequently, it becomes possible for thecomputer, responsible for polling the counters associated with the twogroups of cells in order to monitor the consistency of the indicationsgiven by these counters, to find at a given moment that the indicationsare not identical whereas, if it had polled a very short instantafterwards, it would have found identical indications.

To solve this problem, provision may be made for a delay in validatingthe comparison, that is to say the computer provides an error indicationonly if this error persists for a certain time. However the time forwhich it is necessary to wait is poorly determined since it depends onthe speed of rotation of the button. For a manual control, the time thatit is necessary to wait will be longer if the user turns the button moreslowly. This leads to the fault indication being postponed for exampleby two seconds, this not always being acceptable. Moreover, thisprocedure consists in seeing faults since the computer detects them aspotential faults but in regarding them as false faults for a certaintime even if they are true faults. This solution is not satisfactory.

The invention proposes a different means for monitoring the reliabilityof the information provided by the dual optical angular encoder. Thismeans consists essentially in comparing not the states of the cells ofthe two groups of cells at a given instant but the sequences of at leastfour successive states taken by the two groups of cells before thisinstant and in validating the indications given by the two groups if thesequence for a group is either identical or phase-offset by at most onestate, ahead or behind, with respect to the sequence of the other group.

The encoder making it possible to implement this technique is a dualincremental optical angular encoder, comprising at least one discbearing marks and two pairs of cells for detecting marks, each pair ofcells providing a logic state consisting of a pair of logic levelsallowing the determination of an increment of rotation +1 or −1 when thedisc revolves, this encoder being characterized in that it comprisesmeans for comparing (in principle periodically, by polling) a sequenceof four successive states S0, S1, S2, S3 taken by the first pair ofcells, with a sequence of four successive states S′0, S′1, S′2, S′3taken by the second pair of cells, the last states S3 and S′3 of thesesequences being the states taken at the instant at which the comparisonis made, and means for providing an indication of erroneous counting ifthe sequence S′0, S′1, S′2, S′3 is not equal to S0, S1, S2, S3 or Sx,S0, S1, S2, or S1, S2, S3, Sy, in which Sx represents a prior state ofthe first pair (state immediately prior to the sequence S0, S1, S2, S3)and Sy is a possible state of the first pair such that the increment inpassing from S3 to Sy is not greater than 1 in absolute value.

Other characteristics and advantages of the invention will becomeapparent on reading the detailed description which follows and which isgiven with reference to the appended drawings in which:

FIG. 1 diagrammatically represents the principle of a simple opticalangular encoder of the prior art;

FIG. 2 represents the possible successive logic states of the cells of apair when the disc rotates;

FIG. 3 represents the principle of a dual encoder with monitoring of onechannel by the other;

FIG. 4 represents a chart of the alterations in the states of the twopairs of cells of the encoder of FIG. 3, in the case of a regularrotation;

FIG. 5 and FIG. 6 represent time charts of the alteration of states ofthe pairs of cells;

FIG. 7 represents the architecture of an optical angular encoderaccording to the invention.

Represented diagrammatically in FIG. 1 is the principle of a simpleoptical angular encoder. The encoder comprises a control button 10 thatcan be turned manually and which drives a plane disc 12 bearing marksregularly spaced with an angular pitch P; these marks are preferablyopenings in the disc, the latter being opaque. The width of the openingsis preferably equal to the spacing between the openings, hence, P/2, inthe most interesting case where the encoder makes it possible to encodeby incrementation and by decrementation.

A pair of optical detection cells C1, C2 is used to detect the passageof the marks during the rotation of the disc. These cells C1, C2 arespaced apart by an odd number of half-spacings between marks, that is tosay by an odd multiple (2k+1)P/4 of a quarter of the angular pitch P, kbeing any integer. When the marks are openings in the disc, provision ispreferably made for a light-emitting diode to be placed opposite eachcell, on the other side of the disc with respect to the cell, so thatthe passage of an opening in front of the cell strongly illuminates thecell. After amplification and clipping, the cell provides rectangularsignals visible in FIG. 2. The cell C1 of the pair of cells providesperiodic notches during the regular rotation of the disc. These notcheshave a period T if the disc rotates at constant speed. The cell C2 ofthe pair provides identical notches but out of phase by n/2 or 3 n/2 onaccount of its physical offset (2k+1)P/4 with respect to the cell C1.

There are four possible logic states for a pair of cells, which aresuccessively, if the disc rotates in the direction which corresponds tothe notches of FIG. 2,

-   -   State A: cell C1 at 0, cell C2 at 0: 00    -   State B: cell C1 at 1, cell C2 at 0: 10    -   State C: cell C1 at 1, cell C2 at 1: 11    -   State D: cell C1 at 0, cell C2 at 1: 01

There is no other possible state. After state D we revert to state A.These states are not of interest in themselves; what is of interest isthe transition from one state to another: the transitions from 00 to 10,from 10 to 11, from 11 to 01 and from 01 to 00 all correspond to anincrement of +1, the transition from 00 to 01, from 01 to 11, from 11 to10 and from 10 to 00 all correspond to an increment of −1, that is tosay to a unit rotation in the reverse direction.

A simple encoder 14 analyses these transitions so as to provide a logicsignal T having, in the presence of an actual rotation, two possiblelogic values, the one corresponding to an increment of +1 and the otherto an increment of −1. The signal T can comprise two bits, the oneindicating that there is rotation and the other indicating the directionof rotation, incrementation or decrementation. This signal T is appliedto a counter which counts up or counts down in the presence of arotation.

The control button is provided with indexation slots to avoid thepossibility of it stopping in a position where a detection cell isneither entirely in front of a mark nor entirely between two marks.

If one wishes to improve the reliability with a dual encoder, inprinciple comprising a single disc but two independent detection cellpairs instead of one, the encoder then provides a signal T and a signalT′ representing the successive increments or decrements detected on thebasis of each of the pairs of cells.

The second pair of cells C′1, C′2 is physically offset with respect tothe first (C1, C2) by an integer number of pitches of the marks of thedisc and consequently it provides at the same moment exactly the samestate transitions.

FIG. 3 represents the encoder architecture resulting therefrom: thesignals emanating from the pairs of cells are processed separately andculminate with the separate calculation of the increments T and T′.These increments are compared in a verification circuit 16 before beingsent to the counter which determines the angular position of the controlbutton. If the increments are not identical, then one of the pairs ofcells is operating abnormally and an error signal is emitted.

However, even when the cells are operating normally, the second pair ofcells may not be exactly in phase with the first on account of a slightmechanical offset, the changes of logic state of the cells of the twopairs are not exactly synchronous. As a result, if the verification ofthe identity of the increments is done during the instant at which thepairs of cells do not provide identical indications, the consolidationcircuit is at risk of detecting an error although only a very slightmechanical positioning defect is involved.

FIG. 4 represents the temporal alterations of the state signals whichare derived from the examination of the signals provided by the pairs ofcells C1, C2 and C′1, C′2 during a rotation. The signal Sn representsthe state A or B or C or D of the pair of cells C1, C2 in the course ofa rotation assumed to be at uniform speed. The vertical bars representthe precise instants of changes of state. The signal S′n represents thesame thing for the pair of cells C′1, C′2 in the course of the samerotation. If there is a physical offset that is not rigorously equal toan integer multiple of the pitch of the marks, between the two pairs ofcells, the instants of change of state do not occur rigorously at thesame instants for both pairs of cells even though they regain identicalstates a very short instant afterwards. In the case represented, timeflowing toward the right, the pair of cells C′1, C′2 is slightlyphase-offset ahead with respect to the other. If the rotation wereperformed in the opposite direction, the pair of cells C′1, C′2 would bebehind with respect to the pair C1, C2.

If the verification circuit 16 polls the identity of the transitionsbetween T and T′ at an instant situated during this short phase-offsetand not while the states Sn are quite steady and identical, there is arisk of an error being detected.

Rather than establishing a time constant during which the flaggeddifferences of transition are eliminated, it is proposed according tothe invention that the last four states of the two pairs of cells beobserved and that a validation of the incrementation or thedecrementation be carried out as a function of a comparison of thesestates.

Consequently the succession of states taken by the first pair of cellsis observed. Let S0, S1, S2 and S3 be the last four states, S3 being thelast, corresponding to the state for which one wishes to perform avalidation by comparison with the other pair of cells.

If the cells of the second pair are perfectly in phase mechanically withthose of the first pair, the succession of the last four states S′0,S′1, S′2, S′3 taken by the second pair at the same instant (S′3 beingthe state at the same instant T) is rigorously identical to thesuccession S0, S1, S2, S3 regardless of the instant of comparison. Thisis the ideal situation.

If a slight phase offset occurs, in a configuration such as that of FIG.4, the situation will be the same for any instant of comparison save inthe exceptional case where this instant is situated during the shortphase offset where the states of the pairs of cells are momentarilydifferent.

FIGS. 5 and 6 represent a succession of states Sx, S0, S1, S2, S3, Sy,taken by the two pairs of cells, in two different configurations ofphase offset between cells and at different observation instants t0 inthe two configurations but both taken while the pair of cells C1, C2 isin the state S3.

FIG. 5 represents the case where the pair of cells C′1, C′2 providingthe state S′ is very slightly ahead in phase and the observation instantt0 is situated exceptionally at the very end of the state S3, at amoment where S′ has already toggled from its value S3 to a new value Sywhereas S has not yet done so on account of the slight phase offset. Thevalue Sy depends of course on the fact that the rotation occurs or stopsor reverses. Sy can have only one of the following three values: Sy=S3(rotation interrupted); or Sy different from S3 by an increment+1; or Sydifferent from S3 by an increment −1.

In this case, the sequence of the last four states of S′ before theinstant t0 is S1, S3, S3, Sy whereas the sequence of the last fourstates of S at the same instant is S0, S1, S2, S3.

FIG. 6 represents the reverse case where the pair of cells C′1, C′2 isslightly behind with respect to the pair C1, C2, and the observationinstant t0 lies exceptionally at the very start of the state S3, whereasS′ has not yet toggled into this state S3 on account of the slight phaseoffset.

The sequence of the last four states taken by the pair of cells C′1, C′2before the observation instant t0 is then Sx, S0, S1, S2 whereas thesuccession of states S is S0, S1, S2, S3.

Consequently, if the two pairs of cells operate normally, the successionof states taken by the second pair for a succession S0, S1, S2, S3 takenby the first pair will be:

-   -   in general S0, S1, S2, S3, whether or not there is a slight        phase offset between the pairs;    -   exceptionally S1, S2, S3, Sy but in this case Sy may differ from        S3 only by a positive or negative increment at most;    -   or exceptionally Sx, S0, S1, S2 where Sx is the state preceding        S0 and may likewise differ from S0 only by 0 or +1 or −1.

Hence, according to the invention provision is made for the verificationcircuit 16 to comprise means for comparing at an instant t0 the sequenceof four successive states S0, S1, S2, S3 taken by the first pair ofcells before this instant, with the one sequence of four successivestates S′0, S′1, S′2, S′3 taken by the second pair of cells before thesame instant. This circuit provides an indication of erroneous countingif the sequence S′0, S′1, S′2, S′3 is not equal to S0, S1, S2, S3 or Sx,S0, S1, S2, or S1, S2, S3, Sy, in which Sx cannot differ from S0 by morethan one unit, and Sy cannot differ from S3 by more than one unit.

FIG. 7 represents the encoder according to the invention. A verificationcircuit 20, which may be added or substituted for the verificationcircuit 16, receives the states of the pairs of cells directly. Itsystematically stores at least the last four states of each pair ofcells and validates a transition T bound for the counter only if thesequences of four states comply with what has been stated hereinabove;otherwise it sends an error signal.

To verify whether the sequence of states taken by the second pair is Sx,S0, S1, S2 it is possible to use as value of Sx either the true valuewhich has actually been taken by the pair of cells, but then it has tobe stored in addition to the four states taken by the pair of cells: inthis case five successive states are stored. Or, the fifth state is notstored but the transition is validated only if Sx does not differ fromS0 by more than one unit.

1. A dual incremental optical angular encoder, comprising: a discbearing marks and two pairs of cells for detecting marks, each pair ofcells providing a logic state having of a pair of logic levels allowinga determination of an increment of rotation +1 or −1 when the discrevolves, means for comparing at a given instant a sequence of foursuccessive states S0, S1, S2, S3 taken by the first pair of cells, witha sequence of four successive states S′0, S′1, S′2, S′3 taken by thesecond pair of cells, the last states of these sequences being thestates taken at the instant at which the comparison is made, and meansfor providing an indication of erroneous counting if the sequence S′0,S′1, S′2, S′3 is not equal to S0, S1, S2, S3 or Sx, S0, S1, S2, or S1,S2, S3, Sy, in which Sx represents a prior state of the first pair andSy is a possible state of the first pair such that the increment inpassing from S3 to Sy is not greater than 1 in absolute value.
 2. Theangular encoder as claimed in claim 1, wherein the means for comparingcomprise means for verifying whether the sequence taken by the secondpair of cells is equal to Sx, S0, S1, S2, in which Sx is the state ofthe first cell immediately prior to S0, and these means comprise forthis purpose means for storing a sequence of states of the first pair ofcells comprising the five states prior to the moment at which thecomparison is made.
 3. The angular encoder as claimed in claim 1,wherein the means for comparing comprise means for verifying whether thesequence taken by the second pair of cells is equal to Sx, S0, S1, S2where Sx is any state which differs from the state S0 by at most oneunit.
 4. A method for ensuring the security of operation of a dualoptical angular encoder comprising two pairs of cells for detectingmarks on a disc, these pairs of cells providing logic states whosesuccession determines increments of rotation of the encoder, this methodbeing characterized in that at a given instant the sequences of foursuccessive states taken by the two pairs of cells before this instantare compared and the transition indications given by the two pairs arevalidated if the sequence of four states for a pair is either identicalto the sequence of the other pair, or phase-offset by at most one state,ahead or behind, with respect to the sequence of the other pair.